The Scale of Space

On Monday night I took part in one of my Society’s outreach events for local Cub Scouts. These are a challenge. The Cubs meet around 18:00 and go home to bed before most astronomers would be getting started. It’s a very small window for clear skies in the cloudy and damp UK climate.

We aim for around the first quarter Moon since it’s there in the very early evening, it can take light cloud and the kids love to look at the Moon. I’d observed it on each of the previous three nights, which is not a common occurrence in these parts, but on Monday it wasn’t just cloudy, it was pouring with rain.

So what do you do with a 25 Cubs Scouts on a rainy evening in February? Our normal fallback plan involves Stellarium and a talk about the constellations and planets, but it’s a good idea to get the natives moving occasionally or they get restless!

As the Secretary of the Society I’m kinda expected to take a lead in things. So after a quick sky tour of the constellations and how to find the pole star I decided to try something new to me, and I hoped the Cubs: modelling the solar system.

Collectively we marked out the location of each of the major planets in turn on the hall floor – with sticky labels that are easily removed – on a scale that placed Neptune 15 metres from the Sun. I would have liked to include the Kuiper Belt, but the hall wasn’t long enough, and a smaller scale would have seriously crushed the Cubs representing the inner planets!

These Rocky Inner Planets were all squeezed into the first metre with the Gas Giants in the next four and an awfully lot of space for the Ice Giants. I have to admit that I’ve never laid it all out like this before and the spacing was impressive.

The Sun, which Copernicus pointed out, is at the centre of it all and represents most of the mass in the Solar System. It’s about 109 times the diameter of the Earth, but what really fried a few noodles was the discovery that on this linear scale the Sun is only about 5mm in diameter, or the size of a modest peppercorn!

The Sun to scale in the palm of my hand

That makes the mighty Jupiter around 0.5mm in diameter. I couldn’t find a spice in my cupboard that was small enough for this, and the other planets will be virtually invisible, like fine dust.

It makes you appreciate why it takes so long to travel between the planets. Interplanetary exploration involves a lot of waiting.